How molecular motors start the spliceosome

In order for a cell to produce proteins, it must first translate the instructions encoded in our genes into a readable form. To do this, the gene is translated into a raw version of the messenger RNA (mRNA). However, this pre-mRNA does not yet contain the building instructions in one piece. In a complicated process, the spliceosome has to cut out sections that are not needed and reassemble the information-carrying sections, the exons. This process is called splicing. Splicing has a decisive advantage: if required, the exons can be reassembled in different ways. In humans, around 20,000 genes provide the building instructions for more than 100,000 different proteins.

„The spliceosome is the most complex and dynamic molecular machine in our cells. It consists of more than 150 proteins“, explains Vladimir Pena, who initially led the research work as a group leader at the MPI for Multidisciplinary Natural Sciences and then as a professor at the ICR. „During splicing, the spliceosome undergoes numerous steps in which it changes its structure and composition. These steps are driven by molecular motors called helicases. The complicated structure of the spliceosome makes it very difficult to investigate and understand the function of the machine in detail.“

Into the heart of the splicing oligonucleotide

With the help of cryo-electron microscopy and biochemical methods, Pena's team was able to capture the spliceosome at almost atomic resolution in the middle of the activation process. The focus of this process is a component of the spliceosome, called SF3B1, which is essential for the activation of the machine.

„We have discovered that the spliceosome in humans can only be started with the help of two helicases, PRP2 and Aquarius, by modifying SF3B1“, said Pena. „Helicases are a special type of protein. They convert the chemical energy stored in an ATP - a chemical molecule that provides energy in living cells - into mechanical work. They are therefore autonomous motors that are driven by the ‚ATP battery‘, explains the scientist.

As the researchers were also able to show, PRP2 interacts with SF3B1 in a way that has never before been observed in helicases. Constantin Cretu, one of the first authors of the study, describes the special feature: „Instead of binding to the outer side of the spliceosome like other proteins, PRP2 migrates along the RNA strand to be processed to the ‚heart‘ of the spliceosome. In doing so, it rearranges its structure and composition and puts the molecular machine into an active state.“ The scientists assume that other helicases could also work in this new and unexpected way. In contrast to PRP2, Aquarius performs its work from the surface of the spliceosome.

„With our experiments, we have visualised a crucial new intermediate step in the cycle of the splicing ligand that has not yet been described,

says Jana Schmitzov, also first author of the publication. „As a result, we have uncovered important rearrangements of the proteins: some are sorted out and replaced, others change their position. It is fascinating to see how all the protein players interact with each other in such an orchestrated way.“

Errors in splicing trigger cancer

SF3B1 is the component of the splicing oligosome that is most frequently mutated in some cancers such as leukaemia, uveal melanoma, pancreatic and prostate cancer. This component of the splicing ligosome is therefore an important target for anti-tumour drugs. „We hope that our findings will stimulate new research that will further elucidate the causes of carcinogenesis. These may help to develop cancer drugs that directly target the splicing process,

said Pena.

Article from "idw - Informationsdienst Wissenschaft" from 06/06/2023